1. Field of the Invention
This invention relates to transistors, and more specifically to a trenched DMOS transistor especially suitable for power transistor applications.
2. Description of Prior Art
DMOS transistors are well known. Some DMOS transistors as shown in FIG. 1 are trenched transistors, i.e. the transistor's conductive gate electrode 2 is formed in a trench in a substrate including a drain region 4A and drift region 4B, and the trench walls are insulated with silicon oxide, achieving higher density than that of planar type device. It is well known that trenched DMOS transistors are superior to planar DMOS transistors in terms of having less drain-source on resistance (R.sub.DSON), Cell density is enhanced by reducing the length of the polysilicon gate electrodes 2 by placing them in the trench. The JFET (junction field effect transistor) resistance inherent in a planar DMOS transistor structure is significantly reduced by the gate electrode 2 being formed in the trench. Drain-source on resistance is the resistance between the drain region 4A and the source region 6 when the device is on (conductive) and is undesirable in a transistor.
However it is also known that when cell density is high as in the typical trenched transistor structure, a new undesirable JFET phenomenon gradually appears between the P+ deep body regions 5. The P+ deep body regions 5 typically extend from a principal surface of the semiconductor material into the P body region 7 to provide a contact to the P body region 7. These deep body regions 5 ensure that avalanche breakdown occurs in these regions rather than at the bottom of the trenches. This undesirable JFET phenomenon is because such deep body regions 5 are relatively close to each other. (Also shown in FIG. 1 are conventional drain electrode 8B and source-body electrode 8A.)
Thus while avalanche breakdown occurs rather than destructive breakdown at the trench bottom, i.e. breakdown damaging the insulating oxide at the trench bottom, undesirably this new JFET resistance makes a bigger contribution to drain-source on resistance when cell density is higher.
It would be desirable to reduce the drain-source on resistance of a trenched DMOS transistor at the same time as achieving a high cell density, in order to minimize transistor chip surface area.